The present invention relates to a power supply device and an image forming apparatus using the same and, more particularly, to a belt heating type heating device, a power supply device which can be preferably used for an image forming apparatus such as an electrophotographic apparatus or electrostatic recording apparatus having the heating device as an image heating device, and an image forming apparatus using the same.
Conventionally, there is an image heating device mounted in an image forming apparatus or the like. For descriptive convenience, the prior art will exemplify an image heating device which is mounted in an image forming apparatus such as a copying machine or printer, and heats and fixes a toner image to a printing medium.
Image forming apparatuses widely use heated roller type devices as fixing devices for heating and fixing, as permanently fixed images to the surfaces of printing media, unfixed images (toner images) of target image information formed on and carried by the printing media (transfer sheets, electrofax sheets, electrostatic transfer printing paper, OHP sheets, printing paper, format paper, and the like) by a transfer method or direct method with proper image forming process means such as an electrophotographic process, electrostatic transfer printing process, and magnetic transfer printing process. Recently, belt heating type devices are becoming commercially available in terms of quick start and energy saving. Also, electromagnetic induction heating type devices are proposed. Various fixing devices in image forming apparatuses will be explained.
(a) Heated Roller Type Fixing Device
A heated roller type fixing device is basically constituted by a pair of urging rollers, i.e., a fixing roller (heating roller) and press roller. The pair of rollers are rotated, and a printing medium on which an unfixed toner image to be fixed is formed and carried is introduced, clamped, and conveyed at a fixing nip portion as an urging portion between the pair of rollers. The unfixed toner image is fused and fixed to the surface of the printing medium by heat of the fixing roller and the applied pressure at the fixing nip portion.
In general, the fixing roller uses an aluminum hollow metal roller as a base (core metal), and a halogen lamp as a heat source is inserted in the hollow portion. The fixing roller is heated by heat generated by the halogen lamp, and adjusted in temperature by controlling power to the halogen lamp so as to maintain the outer surface at a predetermined fixing temperature.
Particularly in the fixing device of an image forming apparatus for forming a full-color image which has to have an ability of sufficiently heating and fusing a toner image of four layers at maximum and mixing colors, the fixing roller incorporates a core metal having a large thermal capacity, the core metal is covered with an elastic rubber layer for covering a toner image and uniformly fusing it, and a toner image is heated via the elastic rubber layer. In addition, the pressuring roller also incorporates a heat source, and is heated and adjusted in temperature.
In the heated roller type fixing device, however, even if the power supply of the image forming apparatus is turned on to simultaneously start energizing the halogen lamp serving as the heat source of the fixing device, the thermal capacity of the fixing roller is large, and it requires a long wait time to heat the fixing roller or the like from a cold state to a predetermined fixable temperature, which impairs quick start. The halogen lamp must be energized to maintain the fixing roller in a predetermined temperature-adjusted state so as to execute image formation operating at any time even in the standby state (no image output state) of the image forming apparatus, which increases the power consumption amount.
Especially in a fixing device using a fixing roller having a large thermal capacity, such as the fixing device of the full-color image forming apparatus, a delay occurs between temperature adjustment and temperature rise on the surface of the fixing roller. This causes problems such as a fixing failure, gloss nonuniformity, and offset.
(b) Film Heating Type Fixing Device
Film heating type fixing devices are disclosed in, e.g., Japanese Patent Laid-Open Nos. 63-313182, 2-157878, 4-44075, and 4-204980.
More specifically, a nip portion is formed by clamping a heat-resistant film (fixing film) between a ceramic heater serving as a heating member and a press roller serving as a press member. A printing medium on which an unfixed toner image to be fixed is formed and carried is introduced between the film and the press roller at the nip portion, and clamped and conveyed together with the film. At the nip portion, heat of the ceramic heater is applied to the printing medium via the film, and the unfixed toner image is fused and fixed to the surface of the printing medium by a pressure applied to the nip portion.
The film heating type fixing device can constitute an on-demand type apparatus using a ceramic heater and a low-heat-capacity member serving as a film. Only when the image forming apparatus is to execute image formation, the ceramic heater as a heat source is energized to generate a predetermined fixing temperature. The wait time from power-on of the image forming apparatus to an image formation executable state is short (quick start), and the power consumption in a standby state is very low (power saving). However, this type of fixing device is insufficient in heat quantity as a fixing device for a full-color image forming apparatus or high-speed machine which requires a large heat quantity.
(c) Electromagnetic Induction Heating Type Fixing Device
Japanese Utility Model Laid-Open No. 51-109739 discloses an induction heating/fixing device for inducing a current in a fixing roller by a magnetic flux and generating heat by Joule heat. The fixing roller can directly generate heat by using generation of an induced current, and this fixing device achieves a fixing process with higher efficiency than in a heated roller type fixing device using a halogen lamp as a heat source.
However, since the energy of an alternating flux generated by an exciting coil serving as a magnetic field generation means is used by temperature rise of the whole fixing roller, the heat dissipation loss is large, and the fixing energy density with respect to applied energy is low, resulting in low efficiency.
For this reason, a high-efficiency fixing device is devised by arranging an exciting coil near a fixing roller serving as a heat generation member, or concentrating the alternating flux distribution of the exciting coil in the vicinity of a fixing nip portion in order to obtain energy acting for fixing at high density.
A schematic arrangement of an electromagnetic induction heating type fixing device which concentrates the alternating flux distribution of an exciting coil to a fixing nip portion to attain high efficiency will be described with reference to FIG. 3 used in embodiments of the present invention (to be described later) for descriptive convenience.
In FIG. 3, reference numeral 10 denotes a cylindrical fixing film serving as an electromagnetic induction heat generation rotary member having an electromagnetic induction heat generation layer (conductive layer, magnetic layer, and resistive layer); and 16a and 16b, film guide (belt guide) members whose cross section has an almost arcuated groove shape. The cylindrical fixing film 10 is loosely fitted on the outer surface of the film guide members 16a and 16b. An exciting coil 18 and E-shaped magnetic core (core member) 17, each of which is arranged inside the film guide members 16a and 16b, comprise a magnetic field generation means; and 30, an elastic press roller which forms a fixing nip portion N having a predetermined width with a predetermined applied pressure together with the lower surface of the film guide members 16a and 16b via the fixing film 10, and is urged against the film guide members 16a and 16b. The magnetic core 17 of the magnetic field generation means 15 is positioned in correspondence with the fixing nip portion N.
The press roller 30 is rotated by a driving means M counterclockwise indicated by an arrow. A rotational force acts on the fixing film 10 by the frictional force between the press roller 30 and the outer surface of the fixing film 10 that is generated by rotation of the press roller 30. While the inner surface of the fixing film 10 slides in tight contact with the lower surface of the film guide members 16a and 16b at the fixing nip portion N, the fixing film 10 rotates on the outer surface of the film guide members 16a and 16b clockwise indicated by an arrow at a peripheral speed substantially corresponding to the rotational peripheral speed of the press roller 30 (press roller driving method).
The film guide members 16a and 16b pressurize the fixing nip portion N, supports the exciting coil 18 and magnetic core 17 serving as the magnetic field generation means 15, supports the fixing film 10, and stabilizes conveyance of the fixing film 10 in rotation. The film guide members 16a and 16b is an insulating member which does not inhibit permeation of a magnetic flux, and is made of a material which can bear a heavy load.
The exciting coil 18 generates an alternating flux by an alternating current supplied from an exciting circuit (not shown). The alternating flux is concentratedly distributed at the fixing nip portion N by the E-shaped magnetic core 17 corresponding to the position of the fixing nip portion N. The alternating flux generates an eddy current in the electromagnetic induction heat generation layer of the fixing film 10 at the fixing nip portion N. The eddy current generates Joule heat in the electromagnetic induction heat generation layer by the specific resistance of the electromagnetic induction heat generation layer.
Electromagnetic induction heat generation of the fixing film 10 concentratedly occurs at the fixing nip portion N where the alternating flux is concentratedly distributed, and the fixing nip portion N is heated at high efficiency. The temperature of the fixing nip portion N is adjusted to maintain a predetermined temperature by controlling current supply to the exciting coil 18 by a temperature adjustment system including a temperature detection means (not shown).
As the press roller 30 is rotated, the cylindrical fixing film 10 rotates on the outer surface of the film guide members 16a and 16b. Power is supplied from the exciting circuit to the exciting coil 18 to cause electromagnetic induction heat generation of the fixing film 10. The temperature of the fixing nip portion N rises to a predetermined temperature and is adjusted. In this state, a printing medium P which is conveyed from an image formation means (not shown) and has an unfixed toner image t is introduced between the fixing film 10 and the press roller 30 at the fixing nip portion N with an image surface facing up, i.e., facing the fixing film surface. The image surface comes into tight contact with the outer surface of the fixing film 10 at the fixing nip portion N, and the printing medium P is clamped and conveyed together with the fixing film 10 at the fixing nip portion N.
While the printing medium P is clamped and conveyed together with the fixing film 10 at the fixing nip portion N, the printing medium P is heated by heat generated by electromagnetic induction of the fixing film 10 to heat and fix the unfixed toner image t on the printing medium P. After the printing medium P passes through the fixing nip portion N, the printing medium P is separated from the outer surface of the rotating fixing film 10, and discharged and conveyed.
Inverter circuits used in the electromagnetic induction heating power supply having this arrangement are roughly classified into circuits having a current resonance type power supply method and voltage resonance type power supply method. The resonance method is used to positively generate the vibration state of a voltage or current generated in switching, and switch a switching element when either or both of the voltage and current are low, in order to reduce the loss of a conversion switching element at relatively large power. This method is called soft switching which is the most effective method at large power, and various methods are proposed.
FIG. 16 shows a voltage resonance type inverter circuit as a prior art. In FIG. 16, reference numeral 202 denotes a switching element; 203, a resonant coil (exciting coil); and 205, a resonant capacitor. A known voltage resonance inverter operates such that the switching element 202 is turned on to accumulate power in the resonant coil 203, then the switching element 202 is turned off, and the voltage starts vibration while drawing a resonant arc in a cycle determined by the constants of the resonant coil 203 and resonant capacitor 205. The state at this time is shown in FIGS. 17A, 17B, and 17C.
FIGS. 18A to 18C show operation waveforms when power conversion operation is done by reducing the ON width of a gate switching signal in order to narrow down output power. The voltage waveform of the switching element 202 upon narrowing down output power draws a sine wave which resonates and attenuates with reference to a power supply voltage (level represented by a broken line) connected to the terminal of the resonant coil 203. The vibration amplitude of the voltage depends on exciting power accumulated in the resonant coil (exciting coil) 203, i.e., the ON width of the switching element 202. In power saving, the vibration amplitude is small, the voltage does not satisfactorily drop from the power supply voltage level, and no zero-crossing point is obtained.
More specifically, the switching element 202 switches the load of a very low impedance of the resonant capacitor 205 via the power supply line, and an excessive current flows in switching-on operation. The range where this excessive current does not destruct the switching element 202 can only be narrowed down to about ⅓ the maximum output in the voltage resonant power supply. This makes the design difficult.
However, the above-mentioned prior art suffers the following problem. That is, the mainstream of the required width of a power control region used in a fixing/heating device mounted in an image forming apparatus is 1,100 W to 150 W. Of the current resonant method and voltage resonant method as the induction heating power supply proposed by the present application, the voltage resonant method which can be realized with a simple arrangement is more popular.
However, power control in the conventional voltage resonant method can only narrow down power to about ⅓ the maximum output, i.e., up to 350 W in the above example. If power is to be narrowed down much more, power deviates from the voltage resonant state, and a large current flows through the switching element to destruct it.
If fixing control is done using this power supply in the image forming apparatus, power is excessive at 350 W in the temperature saturation state in continuous printing, and the circuit intermittently operates. This intermittent operation control causes unstable temperature operation. In addition, the voltage resonant circuit is under the load of the parallel circuit of the resonant capacitor and coil, so that an excessive current flows in activation to apply stress to the switching element.
The present invention has been made in consideration of the above situation, and has as its object to provide a power supply device and voltage resonant method which can widen a narrow output control width which is a drawback of a conventional voltage resonant power supply, and can optimize a gate signal waveform to minimize a switching loss in control, which is particularly suitable in adopting as a switching control element a low-cost IGBT (Insulated Gate Bipolar Transistor) which permits a large current, but has a larger loss than an FET.
To achieve the above object, a power supply device for an image forming apparatus according to the present invention is characterized by comprising a first IGBT (Insulated Gate Bipolar Transistor) connected to a power supply, first electricity accumulation means series-connected to the first IGBT, a second IGBT series-connected to the first electricity accumulation means, magnetic field generation means connected between the power supply and a node between the first electricity accumulation means and the second IGBT, second electricity accumulation means parallel-connected to the second IGBT, first rectifying means parallel-connected to the first IGBT, voltage resonant converter means having second rectifying means parallel-connected to the second IGBT, and insulated driving circuit means for shaping a waveform of a transmission voltage transmitted via an output terminal of an insulated transformer, and outputting the transmission voltage as a driving voltage for driving gates of the first and second IGBTs, the insulated driving circuit means having threshold voltage generation means for generating a preset threshold voltage, detection means for detecting that the transmission voltage becomes lower than the threshold voltage when the transmission voltage drops, and first switching means for short-circuiting output terminals of the insulated transformer in response to the detection.
The object also achieved by a power supply device comprising: an alternating current power supply; a rectifier rectifying alternating current generated from said alternating current power supply; a detecting circuit detecting the output voltage of said alternating current power supply; an IGBT (Insulated Gate Bipolar Transistor) switching by the output of said rectifier; and a switching means for short-circuiting the output of said rectifier based on the voltage detected by said detecting circuit.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.